The present invention relates to an encoder for detecting the amount of movement of an object, and more particularly, to an encoder for detecting the steering angle of a vehicle steering wheel.
An encoder that detects the rotational angle of a rotation body is known in the prior art. For example, Japanese Laid-Open Patent Publication No. 2002-98522 describes an encoder that detects the rotational angle of a rotor plate with a plurality of optical sensors. The encoder includes a light-emitting diode (LED), a rotor plate including a plurality of slits formed in the circumferential direction of the rotor plate, and an optical sensor arranged to sandwich the rotor plate with the LED. The optical sensor generates a signal corresponding to the amount of light that passes through the slits. More particularly, the optical sensor generates an ON signal when the amount of light is greater than a predetermined value and an OFF signal when the amount of light is less than the predetermined value. When the rotor plate rotates, the amount of light received by the optical sensor changes synchronously with the rotation of the rotor plate.
However, in the prior art encoder, when the rotor plate stops rotating at a position in which the LED faces toward an edge of a slit (i.e., position in which the amount of light that passes through the slit is in the vicinity of the threshold value of the optical sensor), slight rotation of the rotor plate (unexpected rotation caused by external disturbances) may repetitively generate the ON and OFF signals. Accordingly, the prior art rotor cannot stably detect the rotational angle (movement amount) of the rotor plate (object).